1. Field of the Invention
The present invention relates generally to the field of cable television (CATV) systems and other broadband networks. More specifically, the present invention discloses a broadband network for two-way communications in which the headend terminal monitors the bit error rate of the upstream signal and dynamically adjusts the upstream power transmission level of each remote terminal to maintain a bit error rate below a predetermined limit.
2. Statement of the Problem
A typical CATV architecture employs a tree and branch structure for downstream distribution of video signals. This basic architecture can also be modified to accommodate two-way telephone communications and upstream communication of data for other interactive services. The headend, where program video signals originate, is often connected by fiber optics to a number of optical network units (ONU's). At the ONU, the optical signal is converted to an electrical signal and sent down the distribution network using conventional coaxial cables. This cable is split into different paths for distribution to multiple routes with multiple subscriber units (or remote terminals). If the branches become long enough, bidirectional line amplifiers (also known as trunk amplifiers, distribution amplifiers, bridger amplifiers, or line extenders) are inserted at intervals in the coaxial cable network to boost the signal in both directions. Alternatively, the coaxial cables can also be used to connect the headend to the ONU's, omitting the optical link.
Upstream signals from the remote terminals generally follow the reverse path. Once the return signal from the subscriber unit reaches the ONU, a diplex filter splits the return signal off and routes it through a return optical fiber back to the headend. Presently, most CATV networks transmit signals in the forward or downstream direction at frequencies from approximately 50 to 550 MHz. When the upstream direction is used for upstream telephony and other interactive services, the frequency range is typically in the range of approximately 5 to 40 MHz.
A number of unique problems are associated with maintaining acceptable signal quality while transmitting data upstream over a CATV network due to the ingress of impulse noise and other RF signals. RF interference signals in the 5-40 MHz band include CB radios, amateur radio transmitters, and shod-wave broadcasters. All of these interfering signals tend to be random in nature in at least four ways: duration in time; frequency of occurrence; signal strength; and RF frequency. Two methods are typically used in CATV systems to combat this interference--forward error correction and frequency hopping. Forward error correction utilizes transmission of extra bits in the signal to provide adequate redundancy in the received signal to detect errors and correct them at the receiving end. Several coding schemes exist for error correction, but all have the disadvantage of requiring extra bits to be included with the data. This requires additional error correction circuits at both the receiver and transmitter and additional channel bandwidth, which is a precious commodity in the CATV return spectrum.
Frequency hopping is a method whereby, when the signal quality degrades below acceptable limits, the transmitter and receiver are assigned a new frequency to avoid the interference. If the new frequency also contains interference, the process continues to another new frequency. This method has several disadvantages. First, blocks of open frequencies must be set aside for hopping. This results in reduced overall usage of the return path since at any given time some frequencies must be unused. Second, a complex method of supervision is required to coordinate all transmitters on the return path to ensure that only one transmitter is using a given frequency at any one time. Third, frequency hopping in the middle of a transmission can result in loss of data as the transmitter and receiver stabilize at the new frequency. If framing or a block structure is used in the bit stream, additional data may be lost while the receiver reframes to the incoming data at the new frequency. Fourth, the transmitter must be turned off during the hop to prevent it from interfering with communications in frequency channels between the beginning channel and the new channel. This adds complexity and expense to each transmitter.
A second concern is the summation of noise and ingress in the upstream signal path. The upstream transmission path is unique in that noise from each of the individual legs of the CATV network sums together at the point where the legs join, typically at the tap, power dividers, and other signal distribution devices that serve to split the signal in the downstream direction and combine the signal in the upstream direction. This additive noise and ingress make error-free upstream transmission even more difficult. Typical methods used to combat this summation require the use of RF switches or filters to disconnect upstream legs that do not currently have an actively transmitting upstream device, thus disconnecting the noise and ingress as well. This approach is expensive and is not practical in systems in which many upstream transmitters need to be simultaneously active.
Various other approaches have also been used in the past to reduce upstream noise in CATV systems, including the following:
______________________________________ Inventor Pat. No. Issue Date ______________________________________ Curry et al. 3,750,022 July 31, 1973 Shimp 4,494,138 Jan. 15, 1985 Cilia et al. 4,586,078 Apr. 29, 1986 Dufresne et al. 4,982,440 Jan. 1, 1991 West et al. 5,109,286 Apr. 28, 1992 ______________________________________
Curry et al. disclose a system for minimizing upstream noise in a subscriber response CATV system. If the headend locates a noise source, it commands selected phantom subscriber units in the system to close RF switches to allow only desired upstream transmissions to be passed to the headend and to switch out all other upstream transmission paths not being used.
Shimp discloses a segmented upstream management system for a bidirectional cable television network. Upstream signals from a plurality of subscriber groups are carried by a segmented return-only trunk. The upstream signals for each subscriber group are assigned a frequency band so that groups can be "stacked" for transport. The headend provides down-conversion of the upstream signals.
Citta et al. disclose a CATV system in which upstream signals are transmitted at frequencies that are displaced relative to harmonics of the video frequencies.
Dufresne et al. disclose a two-way CATV network in which narrow-band filters in the distribution lines reduce upstream noise. The filters sense the upstream signal energy and open in the event the energy exceeds a predetermined threshold.
West et al. disclose an example of a reverse path manifold system for combining upstream signals from a plurality of subscribers in a CATV network.
3. Solution to the Problem
None of the prior art references uncovered in the search show a CATV system in which the headend terminal monitors the bit error rate (BER) of the upstream signal from each remote terminal and dynamically adjusts the transmitting power level of each remote terminal to maintain the BER below a predetermined limit.